Method for fixing xerographic images



2,965,482 METHOD FOR FIXING XEROGRAPHIC lNlAGES John H.'Dessauer, Pittsford, Robert W. Gundlach, Spencerport, and George R. Mott, Rochester, N.Y., assignors to Haloid Zerox Inc., Rochester, N.Y., a corporation of New York No Drawing. Filed Sept. 8, 1955, Ser. No. 533,244

4 Claims. c1. 96-1) Thisinvention relates to the field of xerography and, particularly, to an improved method for fixing a xerographic image on its support. t

In the process of xerography, for example, as disclosed in Carlson Patent 2,297,691, issued October 6, 1952, a xerographic plate comprising a photoconductive insulating material on a conductive backing is given a uniform electric charge over its surface and is then exposed to, the subject matter to be reproduced, usually by conventional projection techniques. This exposure discharges the plate areas in accordance with the light intensity which reaches them, and thereby creates an electrostatic latent image on or in the plate coating. Development of the image is effected with a finely divided developing material such as an electroscopic powder which is brought into contact with the coating and is held thereon electrostatically in a pattern corresponding to the electrostatic latent image. Thereafter, the developed xerographic image is transferred to a support material to which it is fixed by any suitable means.

The developing material employed may be of any convenient type suitable to the requirements of a particular application. For example, in line copy applications, the developing material may be of the type disclosed in Walkup Patent 2,618,551, issued November 18, 1952,

wherein a pigmented powdered resin is combined with coated glass bead carriers and is thereby triboelectrically charged to a polarity opposite to that of the electrostatic image to be developed. Subsequently, the combined material is cascaded over the electrostatic latent image whereby the resin particles are caused to adhere electrostatically to the charged portions of the image. Various other pigmented resins, usually having a polystyrene base, are availableto suit specific applications. In continuous tone appli cations of xerography, the developing agent is usually a finely powdered form of carbon that is electrostatically charged and brought into surface contact with the electro static image on the xerographic plate. In such cases the charged carbon particles adhere to the image surface in differing densities depending upon the amount of charge in any given portion of the latent image.

Regardless of the type of development, the developing material particles are held to the support entirely by electrostatic attraction at this stage of the process, and are subject to being smudged during handling. There fore, Whenever an electrostatic latent image is developed to form a xerographic or powder particle image, as indicated above, and such xerographic powder image is then transferred to another support material, it is generally required that the xerographic image subsequently be fixed to the paper or other support on which it is formed. The type of. fixing technique that is employed depends on the requirements of any given installation. For example, the pigmented resins. used for linecopy work may be fixed either by heat fusing or by exposing the image to an atmosphere of solvent for the vparticular type of resin employed. In either case, the powder particles of the image are sufficiently softened to coalesce and to flow United States Patent into the interstices of the paper or support and form a firm bond therewith. Upon cooling of the image or upon the evaporation of the excess solvent, the bonded pigment forms a permanent record of the copy to be re produced.

In the case of continuous tone work, in which powdered carbon is used, the image may be fixed to a support in any of a variety of ways. For example, the powder image may be transferred electrostatically or by mechanical pres sure to an adhesive coated support that is sufficiently tacky to retain the powder, or, the image may be electrostati cally transferred to a plastic coated support that is then heated sufficiently to cause the plastic coating to soften and to adhere to the powder and firmly bind it to the support on cooling, or, the image may be transferred electrostatically to a conventional paper support and then sprayed with Krylon or any other suitable lacquer which, upon drying, is effective permanently to secure the powder particles to the support.

Each of the foregoing methods of fixing xerographic images has been found to be highly suitable for many types of applications and each is in Wide commercial use. However, under certain conditions, it is essential to retain dimensional stability of the support on which the xerographic powder image is to be formed and, also, to avoid coating such supporting material with any substance that may affect the normal utility of the support itself.

A typical example of such a combination of conditions arises in the field of record controlled machines wherein alphabetic and/or numeric characters are perforated in record cards or in tape and serve to control various operations of a wide variety of machines, such as accounting machines, tabulating machines, computers, etc. In this field of endeavor it has been found highly desirable to form xerographic images on the record cards to supple ment or duplicate the perforated data indications and thereby amplify or facilitate the use of such records in their regular applications. However, it has also been found that the conventional xerographic image fixing techniques mentioned above cannot conveniently be employed under all conditions Without occasionally affecting the dimensional stability of the record card. For example, the application of excessive heat or solvent vapor to fix the image can, under certain conditions, result in differential expansion or contraction of portions of the card to an extent to make it unfit for use in its normally intended operation. Similarly, the use of adhesive or plastic coatings on the card could make them difiicult to manipulate during card handling operations. Obviously, any change in the physical characteristics of the record cards that would affect their operability during card sensing, card feeding, or card sorting operations cannot be tolerated.

A convenient solution to such a problem is disclosed in our co-pending application S.N. 515,536, filed June 14, 1955. As disclosed in this co-pending application it is possible to utilize the difference in potential between the residual electrostatic charge remaining on the xerographic powder image after transfer and the charge remaining on the card on which it is formed, by spraying oppositely charged particles of image-fixing materials in the immediate vicinity of the xerographic powder image whereby such fixing material particles are electrostatically attracted selectively to the xerographic powder image rather than to the surface of the card. In the case of line copy applications, the image fixing materials employed may be either of the liquid solvent or low melting point solid solvent type, as desired. In the former case, the charged liquid solvent particles unite with and soften the developing mate rial resin immediately upon contact therewith and cause the resin .to bond firmly to the card. In the latter case, the solid solvent particles are physically coupled to the image particles by electrostatic attraction until they are subsequently heated sufficiently to liquefy the solid solvent particles, whereupon the solvent unites with the developing material resin, as above, to cause the image to be bonded to the card. In the case of continuous tone applications, the image fixing material may be any convenient type of lacquer that functions to coat the image particles and simultaneously pentrates the surface of the card, whereby, when the lacquer solvent volatilizes, the image particles remain firmly bonded to the card. In each of these cases, the xerographic image is fixed in a manner to form a permanent image on the record card without affecting the dimensional stability of the card, and a negligible minimum of the fixing material is deposited on the card surface so that its normal utilization characteristics remain unimpaired.

Although the xerographic powder image fixing techniques disclosed in the above mentioned co-pending application are ideally suited for use in many applications, it has been found that further improvements may be made in the formation of permanent visible images by xerographic techniques whereby the dimensional stability and utilization characteristics of support material may be preserved and, at the same time, the range of materials that may be used may be substantially increased and the ulti-* mate cost involved in the process may be appreciably decreased.

The principal object of the present invention is to provide an improved method for forming a permanent visible image on a support without affecting the dimensional stability or utility of such support. A further object is to provide an improved method for fixing a xerographic image on a support without the requirement for specialized support material. A further object is to provide an improved method for fixing xerographic images on supports that permits the use of low cost image developing material as well as low cost image fixing material. A further object is to provide an improved method for fixing xerographic images that permits the use of a wider range of image forming andimage fixing materials than was heretofore available. A further object of the invention is to provide an improved method for fixing xerographic images on perforated record cards. A further object of'the invention is to improve the fixing of zerographic images in a rapid, economical, and efficient manner.

These and other objects of the invention are attained, in conjunction with xerographic powder images formed on support material, by a utilization of the difference in potential between the residual electrostatic charge re- A maining on the xerographic powder image and the charge remaining on the support on which it is formed shortly after the transfer step, by spraying oppositely charged droplets comprising coloring matter in solution'in a volatile vehicle ind the immediate vicinity of the xerographic powder image, whereby such charged droplets are electrostatically attracted selectively to the xerographic powder image rather than to the surface of the support and wet the powder image and, by capillarity, soak into the surface of the support in the configuration of the powder image. -In this manner the surface of the support is stained, in accordance with the color of the coloring material employed, to form a permanent visible reproduction of the xerographic powder image, and the powder particles of the xerographic image may be brushed off, the card, if desired, as soon as the coloring material vehicle evaporates. By this arrangement, any of a wide variety of materials may be employed as the xerographic developing material. The only limitations on such material is that they be capable of accepting and retaining a triboelectric charge. Additionally, the method of the invention permits the selection of the image forming or fixing material from a wide variety of coloring materials and volatile vehicles, the only requirements being that the coloring matter be sufficiently intense to form a permanent visible image and that the vehicle be capable of accepting an electrostatic charge opposite to .that retained by the xerographic powder image. In employing any of the various materials that may be used in the practice of the invention, the charged droplets of dissolved coloring matter are attracted to the xerographic powder image particles by electrostatic atraction when sprayed in the vicinity of the powder image, and then are attracted by capillary action to the surface of the support in a manner such that a substantial portion of the coloring matter soaks into the interstices of the support material and leaves a visible reproduction of the powder image configuration thereon.

In a conventional application of the process of xerography now in general use, the xerographic plate on which the xerographic powder image is to be first formed is uniformly charged with a positive potential. Upon exposure to the subject to be reproduced, this positive potential leaks otf the surface of the photoconductive layer in the exposed areas of the plate but remains on the image areas of the plate and thereby forms an electrostatic latent image of the subject. During development, the developing material is charged negatively, usually by triboelectric charging, and is brought into surface contact with the plate containing the latent electrostatic image. The negatively charged developing material is electrostatically attracted to and bonded to the positively charged image area of the plate but does not form any electrostatic bond with the exposed area of the plate, so that, after development, a powder particle or xerographic powder image of the subject is electrostatically bonded to the plate.

When the xerographic powder image is transferred to a support material, whether by electrostatic, adhesive, or pressure techniques, as mentioned above, the powdered developing material comprising the transferred image retains substantially all of its negative electrostatic charge. In the case of a pressure or adhesive transfer, the reason for this is quite obvious since there is no step in either process that has any direct effect on the electrostatic charge on the image forming material. In the case of an electrostatic transfer, the support material is placed over the powder image on the xerographic 'plate and is charged to a high positive potential to attract the negatively charged developing material thereto. However, because of the high resistivity of the developing material, as explained below, and the limited surface contact of the developing material with the support material, this step neutralizes only a very small percentage of the original charge on the developing material.

After transfer, in the event the xerographic powder image is not fixed immediately, it is found to remain electrostatically bonded to its support for indefinite periods. Whereas, it has been found that the electrostatic charge placed on the support material bleeds. off and is dissipated in relatively short periods, the negative electrostaticcharge placed on the developing material remains thereon for much longer periods because of the extremely high resistivity of the developing material as compared to the resistivity of the support material. Although specific resistivities of materials such as xer'ographic developing materials and the usual support materials used'therewith are extremely diflicult to measure with any degree of accuracy, empirical datahas indicated that most resin type developing materials in general use, which contain from 5% to 10%. of carbon particles by weight and the remainder in the form of an insulating resin, having resistivities in the range between 10 and IO ohm-cm. On the other hand, most support materials appear to have resistivities of from three to six orders of magnitude below the lowest resistivities of the developing materials. Spe cifically, it can be shown that after transfer of an electrostatic image or after recharging of a transferred image, the potential remaining on the average support material decays to a point that is unmeasurable by conventional electrometer probe. methods in a period of from lO to 20 seconds after transfer, whereas the. image portion of the surface probed maintains a residual potential on the spans order of 20 to 50 voltsduring the sameperiod. In conventional xerographic practices it is well known that a noticeable difference in deposit from a developing material can be observed between areas differing in potential by about one to two volts. Therefore, it is apparent that the above noted potential difference can be utilized to effect selective attraction of oppositely charged droplets of fixing material to' image portions of xerographic records without affecting the non-image portions thereof.

To this end, the method of the present invention comprises the development of the xerographic latent image originally formed on the xerographic plate with a finely powdered developing material of a type capable of accepting and retaining a triboelectric charge, i.e., of a type having a high resistivity, transferring the developed powder image to its support, and exposing the powder image and its support to an atmosphere of oppositely charged droplets of fixing material, such as coloring matter in solution in a volatile vehicle, during the period after image transfer at which the residual charge, if any, has disappeared from the support materal but remains upon the powdered developing material. Preferably, the latter step is accomplished by supporting the support material with image side down in an atmosphere of finely divided droplets of such fixing material so that the only effective forces involved are the electrostatic forces acting between the charged image forming material and the oppositely charged fixing material.

In this manner, the fixing materials are attracted upwardly to the surface of the image material and selectively combine therewith to initiate the fixing action, desired, but are not attracted to the uncharged areas of the support material. The support material is held with image side down so that the fixing material cannot deposit thereon as the result of gravitational action. By this method, the liquid droplets of fixing material wet the powder particles of image forming material as the result of electrostatic attraction, and, as the liquid fixing material wets and encompasses the image forming powder particles, the surface of the support material is also wetted by the fixing material by capillary action, .whereby the surface of the support material immediately underlying the xerographic powder image is colored, in accordance with the coloring material employed in the fixing material, and retains a visible reproduction of the configuration of the xerographic powder image. Thereafter, as the coloring material vehicle volatilizes, the powdered image forming material may be brushed from the surface of the support, if desired.

It may be noted that during the fixing step, described above, the residual electrostatic charge on the image forming material is substantially neutralized by the opposite charge on the fixing material so that the image forming powder may readily be removed from the support. However, the coloring material that soaks into the support is firmly bound in the interstices thereof after the coloring material vehicle volatilizes and thereby forms a permanent visible image of the copy to be reproduced.

By means of the method of the invention, the developing material employed for forming the xerographic powder image may now be selected from a wide variety of substances since it is not essential that it be heat fusible or soluble in organic solvents, as in current xerographic practices. For example, substantially all inorganic pigments are considered usable for such a developing agent. The only rigid requirement being that the selected substance be capable of accepting an electrostatic charge and that it have suflicient resistivity that it retain a portion of such charge for a period of several minutes. Preferably, the substance used should be of such degree of hardness that it will not scratch the surface of conventional xerographic plates during normal use. Typical substances that are considered ideally suited for this purpose are calcium carbonate, zinc oxide, and lead carbo- 6 nate. Obviously, substances such as powdered carbon or pigmented or unpigmented powdered resins may also be used, as in conventional xerogr'aphi-c practices, but these are not considered to be preferred from the standpoint of higher cost. I

The particular fixing agent coloring matter and volatile vehicle may also be selected from wide ranges of substances in accordance with the method of the invention. The only requirement being that the coloring matter be sufficiently soluble in the particular volatile vehicle with which it is employed so that a quantity thereof adequate to form a visible image readily soaks into the surface of the support material during the fixing step of the method of the invention. For example, substances such as crystal violet, malachite green, or rose bengal may conveniently be dissolved in organic solvents such as ethyl alcohol, methyl alcohol, isopropyl alcohol, or trichloroethylene. Alternatively, erythrosin may be dissolved in water for use as the fixing agent. Obviously, a wide variety of comparable fixing agents having the desired characteristics will be apparent to those skilled in the art. In any event, the fixing material is blown or sprayed preferably into an enclosed fixing chamber in which the support material having the xerographic powder image thereon is supported. As the fixing material is injected into the fixing chamber, an electrostatic charge, opposite to the charge on the xerographic powder image, is placed on the fixing material droplets by any suitable charging technique, e.g., induction charging, corona charging, or triboelectric charging, depending uponthe nature of the specific fixing material employed.

The image fixing material is preferably employed in such quantity that the vehicle thereof volatilizes substantially instantaneously as it soaks into the fibers of the support material. However, it is apparent that the support material may be air-dried or subjected to the application of heat to accelerate the evaporation of the vehicle, should a particular combination of fixing and/or support materials so require.

The method of the invention, as described above, contemplates the use of conventional image forming and transferring techniques and commercially available types of image forming and image fixing materials. However, it is apparent that various modifications may readily be employed in the method of the invention. For example, by transferring the xerographic powder image at higher than conventional potentials, residual charges up to volts can be obtained on the powder particles. Also, under certain conditions, it may be possible to re-charge a support on which a xerographic powder image had been formed and thereby produce a higher charge on the powder image. It will be apparent that such modifications are within the scope of the present invention as defined in the appended claims.

Although the method of the invention, as disclosed, is applied to the fixing of Xerographic powder images on perforated record cards, it will be apparent to those skilled in this or related arts that the techniques of the present invention are equally applicable to any situation in which a xerographic powder image is to be made into a permanent visible record, and is particularly applicable in situations in which the dimensional stability or other utilization characteristic of the xerographic image support material is of critical importance.

What is claimed is:

1. In the process of xerography wherein an electrostatically charged xerographic plate is exposed to a projected image to form an electrostatic latent image on the plate corresponding to the projected image and the plate is developed with oppositely charged particles of developing material that are attracted to the latent image to form a xerographic powder image corresponding to the projected image and having a residual electrostatic charge of the same polarity as the developing material and wherein the xerographic powder image is transferred on said support corresponding to the projected image comprising the steps of forming an atmosphere of electrostatically-charged finely-divided droplets of coloring material in solution in a volatile vehicle, the electrostatic charges on said droplets being of opposite polarity to the residual charge on said xerographic powder image, and exposing said xerographic powder image bearing support to said atmosphere, whereby said coloring ma.- terial droplets are selectively attracted to said xerographic powder image.

2, In the process of xerography wherein an electrostatically charged xerographic plate is exposed to a projected image to form an electrostatic latent image on the plate corresponding to the projected image and the plate is developed with oppositely charged particles of developing material that are attracted to the latent image to form a xerographic powder image corresponding to the projected image and having a residual electrostatic charge of the same polarity as the developing material and wherein the xerographic powder image is transferred to a support material capable of absorbing droplets of coloring material, the method of forming a permanent visible image on said support corresponding to the projected image comprising the steps of forming an atmosphere of electrostatically-charged finely-divided droplets of coloring material in solution in a volatile vehicle, the electrostaticcharges on said droplets being of opposite polarity to the residual charge on said xerographic powder image, and exposing said xerographic powder image bearing support to said atmosphere, whereby said coloring material droplets are selectively deposited on the powder particles of said xerographic powder image by electrostatic attraction and a portion of said coloring material droplets are absorbed by the surface of the underlying portions of the support material by capillary action.

3. In the process of xerography wherein an electrostatically charged xerographic plate is exposed to a projected image to form an electrostatic latent image on the plate corresponding to the projected image and the plate is developed with oppositely charged particles of developing material that are attracted to the latent image to form a xerographic powder image corresponding to the projected image and having a residual electrostatic charge of the same polarity as the developing material and wherein the xerographic powder image is transferred to a support material capable of absorbing droplets of coloring material, the method of forming a permanent visible image on said support corresponding to the projected image comprising the steps of forming an atmosphere of electrostatically-charged finely-divided droplets of coloring material in solution in a volatile vehicle, the electrostatic charges on said particles being of opposite polarity to the residual charge on said xerographic powder image, exposing said xerographic powder irnagebearing support to said atmosphere, whereby said coloring material droplets are selectively deposited on the powder particles of said xerographic powder image by electrostatic attraction and a portion of said coloring material droplets are absorbed by the surfaceof underlying portions of the support material by capillary action, and removing the powder particles of said xerographic powder image from the support after the coloring material vehicle evaporates. 4

4. In the process of xerography wherein an electrostatically charged xerographic plate is exposed to a projected image to form an electrostatic latent image on the plate corresponding to the projected image and the plate is developed with oppositely charged particles of developing material that are attracted to the latent image to form a xerographic powder image corresponding to the projected image and having a residual electrostatic charge of the same polarity as the developing material and wherein the xerographic powder image. is transferred to a support material capable. of absorbing droplets of coloring material and having a lower 'resistivity than the developing material particles, the method of forming a permanent visible image on said support corresponding to the projected image comprising the step of forming an atmosphere of electrostaticallycharged finely-divided droplets of coloring material in solution in a volatile vehicle, the electrostatic charges on said droplets being of opposite polarity to the residual charge on said xerographic powder image, exposing said xerographic powder image bearing support to said atmosphere, whereby said coloring material droplets are selectively deposited on the powder particles of said xerographic powder image by electrostatic attraction and a portion of said coloring material droplets are absorbed by the surface of the underlying portions of the support material by capillary action, permitting the volatile vehicle portion of said coloring material droplets to evaporate, and removing the powder particles of said xerographic powder image from the support after the coloring material vehicle evaporates.

References Cited in the file of this patent UNITED STATES PATENTS 

1. IN THE PROCESS OF XEROGRAPHY WHEREIN AN ELECTROSTATICALLY CHARGED XEROGRAPHIC PLATE IS EXPOSED TO A PROJECTED IMAGE TO FORM AN ELECTROSTATIC LATENT IMAGE ON THE PLATE CORRESPONDING TO THE PROJECTED IMAGE AND THE PLATE IS DEVELOPED WITH OPPOSITELY CHARGED PARTICLES OF DEVELOPING MATERIAL THAT ARE ATTRACTED TO THE LATENT IMAGE TO FORM A XEROGRAPHIC POWDER IMAGE CORRESPONDING TO THE PROJECTED IMAGE AND HAVING A RESIDUAL ELECTROSTATIC CHARGE OF THE SAME POLARITY AS THE DEVELOPING MATERIAL AND WHEREIN THE XEROGRAPHIC POWDER IMAGE IS TRANFERRED TO A SUPPORT MATERIAL CAPABLE OF ABSORBING DROPLETS OF COLORING MATERIAL, THE METHOD OF FORMING A VISIBLE IMAGE ON SAID SUPPORT CORRESPONDING TO THE PROJECTED IMAGE COMPRISING THE STEPS OF FORMING AN ATMOSPHERE OF ELECTROSTATICALLY-CHARGED FINELY-DIVIDED DROPLETS OF COLORING MATERIAL IN SOLUTION IN A VOLATILE VEHICLE, THE ELECTROSTATIC CHARGES ON SAID DROPLETS BEING OF OPPOSITE POLARITY TO THE RESIDUAL CHARGE ON SAID XEROGRAPHIC POWDER IMAGE, AND EXPOSING SAID XEROGRAPHIC POWDER IMAGE BEARING SUPPORT TO SAID ATMOSPHERE, WHEREBY SAID COLORING MATERIAL DROPLETS ARE SELECTIVELY ATTRACTED TO SAID XEROGRAPHIC POWDER IMAGE. 